I used my fog light circuit for my "a/c" (a 12v, windshield-suckered fan). I wired my own switched phone charger ports.
I think you should avoid using the fog light circuit for a phone charger. As was said, it will always be off when you start the car. One other minor annoyance is, it will turn off any time your high beams are on. A small thing,but a thing nonetheless.